1. Field of the Invention
The present invention relates to a radio frequency power divider/combiner circuit, and more particularly to a radio frequency power divider/combiner circuit realized by using a microstrip line and lumped elements.
2. Description of the Related Art
In a conventional radio communication system, a power divider/combiner circuit (i.e., divider or combiner circuit) is generally used in a radio frequency (RF) power amplifier. Typically both a divider and a combiner are used--the divider divides an input signal into two or more divided signals, where each divided signal is applied to the input of a separate RF power transistor for amplification; and the combiner combines the output power of the power transistors. Such a power divider/combiner circuit is realized by using a micro-strip line, or a 3 dB hybrid coupler.
The power divider/combiner circuit which is realized by printing a transmission line (e.g., microstrip line) on a substrate, is designed to convert impedance by using a .lambda./4 line (where .lambda. is wavelength). In this case, input and output terminals of a power divider are respectively composed of 50.OMEGA. lines, and the .lambda./4 line is designed as a 70.7 .OMEGA. transmission line to achieve impedance matching. An example of this type of power divider is a "Wilkinson" type power divider.
It is known that the electrical length of a transmission line is determined based on a functional relation of the substrate permittivity and the operating frequency. That is, the physical length of a .lambda./4 transmission line needs to be longer for a relatively lower substrate permittivity and operating frequency. Therefore, it is difficult to realize the .lambda./4 line within a limited space (substrate) in an ultra high frequency (UHF) band. If the power divider/combiner is realized at UHF by using a .lambda./4 line such as in the Wilkinson divider, the power amplifier becomes too large in size for certain applications.
FIG. 1 illustrates a power amplifier which is realized by using a 3 dB hybrid coupler (or a 90.degree. hybrid coupler). In the drawing, an RF signal input from an input terminal INPUT is applied to a hybrid input circuit 110. A resistor R having a resistance of 50 ohms is connected between input terminal ISO and the hybrid input circuit 110. The signals at output terminals, points A and B, of the hybrid input circuit 110 have equal RF power and a 90.degree. phase difference. These signals are inputted to input matching circuits 112A and 112B which provide output signals to transistors 114A and 114B, respectively, which in turn provide output signals to output matching circuits 116A and 116B. In the power amplifier shown in FIG. 1, the current consumption of transistors 114A and 114B becomes different from each other, according to a characteristic, i.e., a return loss of the load. The different current consumption may cause serious damage to one of the transistors having the higher current consumption. As a result, the power amplifier will not operate or will generate decreased output power at terminal OUTPUT of hybrid output circuit 120. A resistor R having a resistance of 50 ohms is connected between the hybrid output circuit 120 and output terminal ISO.
It can be appreciated from the Smith chart shown in FIG. 2 that the current consumption varies according to the characteristic of the load. For instance, consider an impedance locus 202 with an output power being lower by about 1 dB than output power at an optimal point 201 at which the maximum power of the power amplifier is generated, in light of its characteristic. Here, if it is assumed that a reflection coefficient of the output load is a constant, i.e., k, which is generally derived by taking into consideration the resistivity of the load, i.e., .rho..sub.L, the reflection coefficient looking into a point A of FIG. 1, i.e., .GAMMA.A, is represented by .GAMMA.A=k+.theta., where .theta. is the phase of the signal inputted into point A, and the reflection coefficient looking into a point B, i.e., .GAMMA.B, is represented by .GAMMA.B=k+.theta.+180.degree.. For example, if the current has a relationship of I2&lt;I1=I3&lt;I4, where points 1, 2, 3, and 4 represent points on the impedance locus 202 corresponding to the current values I1, I2, I3 and I4, respectively, which show the variation of current consumption as a function of the load, as shown in FIG. 2, the reflection coefficient .GAMMA.A corresponds to a position (4) and the reflection coefficient .GAMMA.B corresponds to a position (2). As a result, the transistor 114A (see FIG. 1) has the minimum current consumption and the transistor 114B (see FIG. 1) has the maximum current consumption, thereby resulting in the maximum current consumption difference between the two transistors. Then, a junction temperature (hot spot) of the transistor 114B increases, which results in serious damage to the transistor 114B.
As described above, if the power amplifier is realized in microstrip line, the power amplifier increases in size undesirably at lower operating frequencies such as the UHF band. Further, if the power amplifier is realized by using the 3 dB hybrid coupler, the outputs and the current consumption of the transistors 114A and 114B of the power amplifier vary according to the load characteristic (i.e., the reflection coefficient).